This invention is directed to an apparatus by which solder thickness is measured by use of a probe to take a base measurement on the solder surface, liquefying the solder, and taking a second position measurement, the difference of which is the solder thickness, thus providing a non-destructive measurement system.
Solder is put on the copper traces and pads on printed wiring boards to facilitate the attachment of electrical devices, such as leadless chip carriers, flatpacks, and axial leaded components. Electroplating is the preferred process to place solder on the traces and pads. The control of solder plating thickness has always been accomplished by calculating the process parameters to achieve the desired solder plating thickness. The plating takes place along those process parameters, and the desired solder plating thickness is presumably achieved. In order to check the thickness, an adjacent test coupon is plated at the same time and presumably with the same conditions as the circuit traces. However, there is often an error in the solder plating thickness on the coupon as compared to the circuit traces due to variations of current flux density over the printed wiring board. The thickness of the solder plated on the coupon is measured by microsectional analysis. Such analysis takes time, and feedback information is not always readily available to correct plating errors.
X-ray fluorescence is a non-destructive method for measuring solder thickness. One disadvantage of X-ray fluorescence as a measuring method is that it can only be employed to measure solder thickness less than 0.002 inch. While such solder thickness is suitable for many applications, the use of a thicker solder layer is desirable for mounting a leadless chip carrier. When a leadless chip carrier of moderate size is supported by a relatively high column of solder, the solder will not suffer from cracking when subjected to mechanical stress caused by the mismatch in the thermal expansion coefficients between the printed wiring board and the ceramic leadless chip carrier. A tall column of soft solder can flex to absorb the stress. In such applications, a measuring procedure which is capable of measuring thickness in excess of 0.002 inch is required.
Beta particle backscatter is another solder thickness measurement system used in the circuit board industry. In this procedure, a high energy beta ray emitter such as strontium-90 is employed. This procedure is capable of measuring solder thickness up to 0.005 inch. However, this solder thickness measuring procedure gives varying results with changes in composition of the plated solder alloy. Similarly, the indicated thickness measurement changes with the presence of copper layers within the printed wiring board. Due to measurement uncertainties, this method is not commonly used.
Accordingly, a solder thickness measuring system and apparatus is required which is non-destructive of the solder plate and printed wiring board, and which allows for timely and economical solder thickness measurement with an accuracy better than the present-day systems. In addition, there is need for such a system and apparatus wherein the solder thickness does not limit the measuring accuracy. Further, such a solder thickness measurement system and apparatus should not be affected by variations in solder alloy or variations in printed wiring board substrate composition.